Effect of transition metal substitution on high-temperature properties of thermoelectric materials

Güneş, Murat
The objective of the present dissertation is to improve the thermoelectric efficiency (ZT) of Ca3Co4O9 metal oxide with a focus on nanostructuring, porosity and elemental substitution altering the electrical and thermal transport properties at high temperature and to establish a unique high temperature Seebeck coefficient and electrical resistivity measurement system. An instrument for the high temperature measurement of Seebeck coefficient and electrical resistivity of one and/or two bulk samples over a temperature range of 300 – 1100 K is designed and implemented in our laboratory. During the installation of the system, five inventions have been made. These are; 1) cold-finger effect eliminated triple beadless thermocouples, 2) magnetic field and furnace eliminated micro-heaters, 3) differential temperature measurement specialty, 4) two-sample simultaneous measurement feature 5) non-destructive axial four-point probe electrical resistivity measurement conduction. Due to the above mentioned major accomplishments, while cost and size is reduced, measurement accuracy and sensitivity is further enhanced. The total estimated data error for the Seebeck coefficient and resistivity measurements is less than 2.6 % and 1 %, respectively. In order to investigate the effect of nanostructuring and porosity on thermoelectric properties of Ca3Co4O9 oxide, samples of the material are synthesized via citrate sol-gel method with different average grain sizes ranging from 18, 30, 68 nm and 2 m. A considerable increase in electrical resistivity is observed, however, Seebeck coefficient is not much suppressed. It is investigated that thermal conductivity is substantially lower for nano-sized samples due to grain boundary scattering. Porosity further contributes to reducing this value leading to increase in the density of scattering regions. Thermal conductivity is decreased by 23 % with the help of nanostructuring and porosity. The enhancement of ZT value is 61 % and it reaches 0.29 at 1000 K for 18 nm sample. Polycrystalline Ca3-xCrxCo4O9 (x = 0.0, 0.01, 0.03, 0.05 and 0.07) thermoelectric samples are prepared. Cr substitution to Ca-site leads to the spin-entropy enhancement and this caused increase in the Seebeck coefficient with up to x = 0.03 Cr substitution. Power factor is enhanced by 30 %. The thermoelectric performance is the most significant in x = 0.03 sample with ZT value of 0.26 at 1100 K. The enhancement in ZT value is 33 %.